A method for gene transfer to plants via Agrobacterium, a soil bacterium (Agrobacterium method) can be carried out by convenient procedures without the need of special techniques or apparatuses. The Agrobacterium method can transfer a gene in a complete form and with a small copy number to a plant. This method is therefore used with high frequency in the preparation of transformed plants, as compared with other plant gene transfer methods. However, on the other hand, there are many plant species having low transfer efficiency by the Agrobacterium method. Accordingly, the development of Agrobacterium having enhanced gene transfer ability to plants has been demanded.
Super-Agrobacterium has been reported, which is Agrobacterium having improved gene transfer efficiency to plants by harboring a plasmid containing an ACC deaminase gene imparting the ability to inhibit ethylene production to Agrobacterium (Patent Literature 1 and Non Patent Literature 1). The development of Agrobacterium having high gene transfer efficiency has still been demanded in order to improve gene transfer efficiency on the basis of various mechanisms. The present inventors have developed a technique of stably improving the gene transfer efficiency of Agrobacterium by integrating an ACC deaminase gene into a predetermined region on the genome of Agrobacterium (Patent Literature 2).
Meanwhile, GABA (γ-aminobutyric acid) is produced in plants as a result of damages to or Agrobacterium infection of the plants (Non Patent Literature 2). Plants highly producing GABA have also been reported to have low susceptibility to infection by an Agrobacterium tumefaciens C58 strain (Non Patent Literature 3). Plant GABA is further known to inhibit the quorum sensing signals of Agrobacterium and inhibit the replication of Ti plasmids necessary for gene transfer or horizontal transfer among Agrobacterium bacteria (Non Patent Literatures 4 and 5). It is, however, unknown whether the GABA activity of plants can be altered by modification of Agrobacterium and whether gene transfer efficiency can thereby be changed.